In the past, so-called pin-barrel extruders were always used as discharge and homogenizing extruders, such as are known, for example, from German Offenlegungsschrift No. 2 235 784 or German Offenlegungsschrift No. 3 003 615 belonging to the assignee of the present invention. In extruders of this type of construction, metal pins protrude radially through the extruder housing into the processing chamber of the extruder, the extruder screw having interrupted screw flights in this region.
These extruders are distinguished by their very high discharge rate and good homogenizing effect upon the material to be processed and permit also an increased throughput of material per unit of time, compared with conventional cold-feed extruders having a screw with a shearing section, with the r.p.m. of the screw remaining constant.
These advantages have resulted in pin-barrel extruders becoming the most commonly used extruders in the rubber industry in the last 15 years.
Independently of this, a mixing section for an extruder has been developed, which has become known as a transfer mixing section (DE-A 11 42 839). This mixing section is substantially characterised in that both the extruder screw and the internal wall of the extruder housing are provided with grooves and flights over a predetermined length, the thread depth of the extruder screw, when viewed in the longitudinal direction of the extruder housing, decreasing to zero and subsequently increasing again at the same rate as the thread depth of the grooves in the housing respectively increases and decreases again. As a result of this configuration for the extruder screw and housing, the extruded material can be exchanged fully between the screw grooves and the housing grooves, thereby producing a good mixing effect.
Compared with the pin-barrel extruder, the transfer extruder could claim for itself a certain corner of the market, especially when the overall length of the extruder had to be kept small.
It is additionally known from U.S. Pat. No. 3,613,160 to provide extruders with throttle components, whereby the conveyance of extruded material in the extruder may be variably controlled externally. For this purpose, according to this publication, a substantially cylindrical component is disposed on the screw shaft of the extruder screw, said component rotating jointly with the screw and completely blocking the processing chamber downstream. In the region of this cylindrical component, two throttle pins each extend, externally, through the extruder housing radially into an axially oriented overflow conduit, which is incorporated into the internal wall of the extruder housing.
When the throttle pins are retracted, a portion of the extruded material situated upstream of the cylindrical component may pass through these conduits to the downstream section of the extruder. This flow of extruded material can be controlled by the insertion of the throttle pins into these overflow conduits to different depths.
Finally, a transfer extruder is known from German Patent Specification No. 27 31 438, wherein the number of flights in the housing and on the screw in the transfer section decreases as the cross-sectional area of the screw threads increases and increases as the cross-sectional area of the screw threads decreases. In such case, screw threads having a larger cross-sectional area have a greater width than screw threads having a smaller cross-sectional area.
The tests which were carried out in the past on such mixing sections have indeed shown that comparatively good mixing qualities are achievable therewith, and that the mixing section is completely self-cleaning. However, it was disadvantageous that the type of construction proposed resulted in a relatively large manufacturing outlay and, in consequence, the production costs were high.